1. Field of the Invention
The present invention generally relates to optical lenses for light emitting devices and, particularly, to an optical lens typically used for a side emitting light-emitting diode (LED).
2. Discussion of the Related Art
Nowadays, LEDs are widely applied in electronic display devices and illuminating devices. This is because LEDs typically offer the advantages of high illuminating efficiency and a long working lifetime. An LED generally includes a semiconductor chip for emitting light. LEDs can be classified into two kinds according to the location of the semiconductor chip therein: bottom emitting LEDs, and side emitting LEDs.
Referring to FIG. 1, in a typical bottom emitting LED, a semiconductor chip 11 is disposed below a display screen 12. The semiconductor chip 11 is configured for emitting plural kinds of light colors, for example, red, green, and blue (RGB). A distance D1 between the semiconductor chip 11 and the display screen 12 must be large enough to provide a predetermined threshold angle for the RGB lights emitting from the semiconductor chip 11. Thereby, the emitting RGB lights can be adequately mixed and yield white light that illuminates the display screen 12. The distance D1 is apt to increase a thickness of the bottom emitting LED, thereby increasing the overall size of the bottom emitting LED.
Referring to FIG. 2, in a typical side emitting LED, a display screen 22 is stacked on a light guide plate 24, and a semiconductor chip 21 is disposed on at one side of the combined display screen 22 and light guide plate 24. Light emitted from the semiconductor chip 21 travels along light paths including light paths 23 (only one shown). The light paths 23 are located within the light guide plate 24, so that the light can be reflected time after time until the light exits a top of the light guide plate 24 and thus illuminates the display screen 22. Therefore the side emitting LED can provide improved uniformity of light that illuminates the display screen 22. However, an amount of light energy may be lost upon each reflection, and thus the side emitting LED has a limited efficiency of utilization of light. In addition, in the case of a large size display screen 22, some areas of the display screen 22 may not be sufficiently illuminated.
Referring to FIG. 3, this shows an optical lens 31 that is used in another kind of typical bottom emitting LED. The optical lens 31 is configured to improve the efficiency of utilization of light. The optical lens 31 includes a base portion 32, a top reflecting surface 34, a peripheral first refracting surface 36 obliquely angled with respect to a central axis 35 of the optical lens 31, and a peripheral, curved second refracting surface 38 extending from a bottom of the base portion 32 to the first refracting surface 36. The base portion 32 defines a bottom cavity (not labeled) therein. A bottom surface of the base portion 32 is shaped like a flat-topped dome. A semiconductor chip (not shown) can be disposed in or below the bottom cavity. Typically, the semiconductor chip emits light from a point “F” as shown. Light entering the optical lens 31 through a central flat portion of the bottom surface of the base portion 32 in the cavity propagates to the reflecting surface 34. The light is reflected by the reflecting surface 34 to the first refracting surface 36. The light is refracted by the first refracting surface 36, and exits the optical lens 31 in a direction substantially perpendicular to the central axis 35. Light entering the optical lens 31 through a peripheral curved portion of the bottom surface of the base portion 32 in the cavity propagates to the second refracting surface 38. The light is refracted by the second refracting surface 38, and exits the optical lens 31 in a direction substantially perpendicular to the central axis 35.
The optical lens 31 may be employed in side light-emitting devices, so that the side light-emitting devices may be advantageously used with light guides and reflectors that have very thin profiles and/or large illuminated areas. However, each of the first refracting surface 36 and the second refracting surface 38 is a single smooth peripheral surface. Thus it is difficult to manufacture the optical lens 31 to have desired light distribution characteristics and optimum light emitting angles. In addition, if the light incidence angle at the central flat portion of the bottom surface of the base portion 32 is not within a predetermined range, the light may escape from the optical lens 31 through the reflecting surface 34 instead of being reflected to the first refracting surface 36. When this happens, the efficiency of utilization of light is reduced.
Therefore an optical lens which can overcome the above-described shortcomings is desired. A light emitting device employing the optical lens is also desired.